Direct powder rolling



United States Patent This invention relates to a method of direct rolling of atomized pellets of magnesium-base alloy into sheet or strip form, and to a method of preparing spheroidal atomized pellets of magnesium-base alloy for the direct rolling step, as well as to the deformed spheroidal pellets.

For the purposes of the specification and claims, a magnesium-base alloy is defined as an alloy consisting of at least 75 percent by weight of magnesium. V 1

Atomfzed pellets are to be understood asp'ellets of the alloy prepared directly from the molten metal, as by jet or disc atomizing, and as contrasted toground powder 7 prepared from solid metal. I

Spheroidal atomized pellets are defined as those having a diameter to Width ratio of from 1 (spheroidal pellets) to a value less than 1.5.

The term sheet is meant to include both sheet and strip. Heretofore, difficulties have been encountered on attempting to roll magnesium-base alloy powder into sound sheet. Ground powders tend to acquire more than a light oxide coating during preparation and handling, tend to be moreor less contaminated by the grinding surfaces,

, and the ground powder is both more expensive and more hazardous to make than the atomized pellets. The atomized pellets, on the other hand, While far more free of contamination, tend to be spherical, or so nearly spherical as to be more properly termed spheroidal, having mainly a diameter to width ratio between 1 and a value less than 1.5.

lf spherical to spheroidal pellets of magnesium-base alloy, having the temperature of the ambient surroundings, are passed through the rolls of a rolling mill, also at ambient room temperature, it is found that the pellets are not pressed and welded together into sheet. Sheet simply is not formed. 0n attempting to hot roll these spherical to spheroidal pellets into sheet form, it is found that the pellets tend to rotate on the roll surfaces and,

there-fore, are not readily gripped by and drawn between the rolls. A steady passage of pellets through the rolls is required to produce sound sheet. This steady passage of pellets cannot be obtained when the feed material i consists of spherical to spheroidal pellets.

It is, therefore, a principal object of the invention to provide a method of preparing sound sheet from spheroidal atomized pelletsfof magnesium-base alloy.

An additional obiect of the invention is to provide a methodof transforming spheroidal atomized pellets of magnesium-base alloy into suitable feed material for direct powder rolling.

Another object of the invention is to provide modified atomized pellets of magnesium-base alloy whichv are suitable as feed materim for direct powder rolling into sheet form.

These and other objects and advantages of the present invention will be more clearly understood on becoming familiar with the following description and the appended claims.

The invention is predicated on the discovery thatupon passing spheroidal atomized pellets of magnesium-base alloy between the rolls of a mill while the spheroidal pellets and the rolls are at a temperature below that at which the pellets substantially stick together onbeing rolled, and above the temperature at which the pellets fracture readily, and with a nominal roll clearance of substantially zero, the atomized pellets are flattened; and

. 3,17%,516 Patented Apr. 2o, lees atomized pellets formed in an inertatmosphere tend to be very nearly spherical or spheroidal, and are substantially free of oxide contamination on the surface of the pellets. Examination of these pellets, which vary from spherical tooblate or spheroidal, shows that most of them have a diameter to width ratio in the range of 1 to less than 1.5. p

In other words, the ratio of the longest dimension to the shortest dimension per pellet, respectivel is generally in the range of 1 to 1.5. While some pellets more irregular in shape than this may well be produced in the atomizing process, in addition to the more nearly spherical pellets, it is the more nearly spherical pellets which cause the major difficulty in direct powder rolling.

It has now been found that it is necessmy to pass the atomized pellets between the rolls of a mill and flatten the more nearly spherical pellets in order to transform the pellets into suitable feed material for direct powder rolling into sound, fully densified sheet metal. In carrying out the pellet-flattening step, it is desirable to flatten the pellets enough so that the width to diameter ratio of the typical pellet is at least 1.5 and preferably at least 1.7. This should be done without causing groups or clusters of pellets to become stuck together, and in addition, since it is not desirable to produce exceedingly fine particle size material, fracturing of the pellets should be in the range or" 2.5 to 15 feet per minute is usually satisfactory. The pellets and the rolls are maintained at a temperature in the range of room temperature to about C. The exact temperature employed varies some what with the nature of the magnesium-base alloy in order to achieve the stated objective of flattening the pellets without causing groups of them to adhere together, but at the sametime, avoiding fracturing the pellets.

The flattened pellets prepared as feed material for directpowder rolling are preheated immediately prior to rolling into sheet. perature inthe range of 350 to 550 C., or more, preferably in an inert atmosphere. Time at temperature is not necessary. It is adequate to bring the pellets to the desired temperature. will vary somewhat with the nature of the magnesiumbase alloy and according to the metallurgical effects it is desired'to obtain during the rolling process.

The-preheated pellets, while they are at the desired elevated temperature, are gravity-fed between the rolls of a mill at a roll speed in the range of 8 to about 15 feet per minute. The rolls are preheated to a temperature generally in the range of about 320 to 400 C., or more,

. care being taken to avoid too high a temperature or pres- 7 must have substantiallyzero clearance, and more usually,

The pellets are heated to a tern The exact temperature employed pressure is applied; for example, the rolls may be drawn apparent angle of nip at which the powder is seized by the rolls of the mill and passed therebetween. The angle of nip is the angle defined by the tangents to the points on the rolls (in transverse view) at which there is substantial friction between the powder and the roll. The angle of nip is a function of particle shape, roll and powder identity, and the temperature and clearance between the rolls, as well as the roll diameter. If the angle of nip is large, the metal is seized more quickly by the rolls and is subjected to more working in the rolling process than is metal vw'th smaller angle of nip characteristics. Pellets which are more irregular in shape give rise to a larger angle of nip onbeing rolled. In addition, increasing the temperature of the pellets and of the rolls and decreasing'the clearance between the rolls, all appear to. increase the angle of nip.

The reduction ratio is defined as the thickness of a differential element at apoint between the rolls corresponding to the angle of nip divided by the final strip thickness. A reduction ratio of atleast about 3.5, and more preferably at least 4, is necessary in order to obtain sound sheet. Powders giving rise to nip angles less than about 2 cannot be satisfactorily rolled into thin strip, even that as thin as 0.010 inch. Preferably, the requisite conditions, including particle shape irregularity, are em ployed that a nip angle of at least about 4 obtains. Some typical nip angles are listed in the following Table I.

TABLE I Nip angles for several types of magnesium alloy powders as determined by strip thickness measurements after rolling powders cold Type of powder: Nip angle, degrees powder to the rolls by forced feeding, e.g., by a screw feeding arrangement. Forced feeding thus reduces the requirements such as roll-diameter and particle shape irregularity.

As an example of the pellet flattening step in the process of the invention, spherical pellets formed of AZ31B magnesium-base alloy having a diameter to width ratio substantially'equal to 1,-and-the-following sieve analysis.

were dumped in a continuous stream directly between the rolls of a vertically (downwardly) fed rolling mill having 6 inch diameter rolls, 6 inches long. "The rolls were operated at a roll speed of 15 feet per minute and with zero clearance between the rolls. The rolls and the feed material were at room temperature. The emerging distorted, i.e., partially'flattened, pellets exhibited, typi- I cally, a diameter to width ratio of about 1.7. A sieve analysis of the flattened pellets showed that 41 percent by weight of the pellets were retained on a No. 20 sieve, as contrasted with only 3.1 percent retained before flattening. The unsieved powder is generally used as feed material to be rolled directly into sheet or strip, although a sieved fraction, such as the fraction of the flattened pellets retained by a No. 20 sieve or the fraction passing a No. 20 sieve, may be used if desired.

Flattened pellets of AZ31B magnesium-base alloy, flattened as described above, were subjected to direct powder rolling under various conditions of preheat temperature, and, roll temperature and clearance. The rolling conditions and the ultimate tensile strength (U.T.S.) and elongation (E) properties are listedin Table III. Roll speed was about 10.5 feet per minute.

By way of comparison, several attempts were made, under varied conditions, 'to make sheet material from unflattened pellets'of AZ31B alloy. The conditions and results are summarized in Table III.

TABLE III Data for direct powder rolling of various magnesium alloy powders Condition of A2313 Powder R011 R011 gap, Roll Strip U.I.S., Elonga- Test No atomized pellets preheat temp., inches torque thickness, p.s.i. on,

' temp., 0 0. inches percent Flattened, not sieved 500 240 0.027 2 18,100 1 0 Flattened, 20 mesh-.- 525 370 0. 022 1 23, 700 2 2. 5 Flattened, +20 mesh 500 355 0.033 1 22, 800 2 1.8 do 525 500 0. 048 9 25, 500 2 4. 3 Flattened, not sieved. I 525 300 0. 025 1 26, 000 2 4. 0 do 526 320 0. 024 1 27, 800 2 6. 4

Spherical, as-received 525 320 1 d0 525 440 425 240 525 300 1 Average values, 20 mesh =passed No. 20 Sieve.

2 Maximum values, +20 mesh =retained on No. 20 Sieve.

Wrench' tight 1 No strip formed.

As an example of the roll diameter relationship, using a powder giving rise to a nip angle of 2, a roll diameter of inches would be required to produce 10 ml strip. On'the other hand, to produce thick strip, e'.g., 0.25 inch in thickness, with rolls 4 feet in diameter, a powder should be employed with exhibits a nip angle of about 10, based on a gravity feeding process.

The apparent nip angle may be increased for a given powder, if desired, by increasing the pressure of the roll pressure must be employed to produce strip from AZ31B pellets equivalent in ultimate tensile strength to strip formed of ZK pellets.

6 preheating the so-flattened pellets to a temperature in the range of about 350 to 550 C.; and

feeding the preheated, flattened pellets between the TABLE IV Powder Roll Test No. Type of Feed Material preheat temp., R011 opening U.T.S., Elongation,

tan-3)., C. p.s.i. percent 240-1) Am llapcllets flattened all 5 397 Wrench fighL 234-1) Zlll ll pellets flattened at 525 331 0 351000 7. 5

1 RIP. =room temperature. 2 Front of strip. 3 Back of strip. Among the advantages of the process of the invention are that it can be operated in either a batch-Wise or continuous manner, and the sheet produced exhibits desirable high physical strength properties in accordance with the fine-grained nature of the solid metal so-produced.

Although the invention has been described and specific examples given, it is apparent that many variations in the process may be made without departing essentially from the nature of the invention as may be encompassed in the following claims.

I claim: 1. The method of transforming spheroidal, atomized pellets of magnesiumbase alloy into sheet form which comprises:

passing the spheroidal, atomized pellets between the rolls of a mill while the spheroidal pellets and the rolls are at temperatures below that at which the pellets substantially stick together on being rolled, and with a nominal roll clearance of substantially zero, thereby to flatten the spheroidal pellets; preheating the so-flattened pellets to a temperature in therange of about 350 to 550 C.;

feeding the preheated, flattened pellets between the heated rolls of a mill, the rolls having substantially zero clearance, and the rolls being at a temperature in the range of about 320 to 400 0, thereby to form sheet material of magnesium-base alloy.

2. The method of transforming spheroidal, atomized pellets of magnesium-base alloy into sheet form which comprises:

passing the spheroidal, atomized pellets between the rolls of a mill, while the spheroidal pellets and the rolls are at temperatures in the range of room temperatnre to about 150 C., but below the temperature at which the pellets substantially stick together on being rolled, and with a nominal roll clearance between zero and a value substantially less than the diameter of the average spheroidal pellet before rolling, thereby to flatten the spheroidal pellets; preheating the so-flattened pellets to a temperature in the range of about 350 to 550 C.; and

- feeding the preheated, flattened pellets between the heated rolls of a mill, the rolls having a clearance therebetween of 0.01 inch to wrench tight and the rolls being at temperatures in the range of about 320 to 400 0., thereby to form sheet material of magnesium-base alloy. 7

3. The method of transforming spheroidal, atomized pellets of magnesium-base alloy into sheet form which comprises:

passing the spheroidal pellets between the rolls of a mill, while the spheroidal pellets and the rolls are at temperatures below that at which the pellets substantially stick together on being rolled but above the temperature at which the pellets fracture on being rolled, and with a nominal roll clearance between zero and a value substantially less than the diameter of the average spheroidal pellet before rolling, thereby to flatten the spheroidal pellets;

heated rolls of a mill, the rolls having a clearance therebetween of 0.01 inch to wrench tight and the rolls being at temperatures in the range of about 320 to 400 0, thereby to form sheet material of magnesium-base alloy.

4. The method of preparing spheroidal, atomized pellets of magnesium-base alloy for subsequent direct powder rolling which comprises:

passing the spheroidal, atomized pellets between the rolls of a mill while the spheroidal pellets and the rolls are at temperatures below that at which the pellets substantially stick together on being rolled, and with a nominal roll clearance of substantially zero, thereby to flatten the spheroidal pellets.

5. The method of preparing spheroidal, atomized pellets of magnesium-base alloy for subsequent direct powder rolling which comprises:

passing the spheroidal, atomized pellets between the rolls of a mill, while the spheroidal pellets and the rolls are at temperatures in the range of room temperature to about C., but below the temperature at which the pellets substantially stick together on being rolled, and with a nominal roll clearance between zero and a value substantially less than the diameter of the average spheroidal pellet before roling, thereby to flatten the spheroidal pellets.

6. The method of preparing spheroidal, atomized pellets of magnesium-base alloy for subsequent direct powder rolling which comprises:

passing the spheroidal pellets between the rolls of a mill, while the spheroidal pellets and the rolls are at temperatures below that at which the pellets substantially stick together on being rolled but above the temperature at which the pellets fracture on being rolled, and with a nominal roll clearance between zero and a value substantially less than the diameter of the average spheroidal pellet before rolling, there by to flatten the spheroidal pellets.

7. In a method of rolling spheroidal, atomized pellets of magnesium-base alloy into sheet form in which the spheroidal, atomized pellets are preheated and passed through the heated rolls of a mill and substantially zero clearance therebetween, the improvement which cornprises:

before the preheating step, passing the spheroidal,

atomized pellets between the rolls of a mill while t e spheroidal pellets and the rolls are at temperatures below that at which the pellets substantially stick together on being rolled, and at a temperature above that at which the pellets fracture on being rolled, and with a nominal roll clearance of substantially zero, whereby the average rolled pellet exhibits a diameter to Width ratio of at least 1.5.

8. In a method of rolling spheroidal, atomized pellets of magnesium-base alloy into sheet form in which the spheroidal, atomized pellets are preheated and passed through the heated rolls of a mill and substantially zero 7 clearance therebetween, the improvement which com- Relerences Cited by the Examiner prism UNITED STA E" PATENT before the preheating step, passing the spheroldal, b S

atomized pellets between the rolls of a mill While he 11/53 Lcntls 75 214 spheroidal pellets and the rolls are at temperatures 5 2796660 6/57 llmann 75 214 below that at which the pellets substantially stick to- FOREIGN PATENTS gether on be1ng rolled, and at a temperature above 799L973 8/58 Great Britain that at which the pellets fracture on being rolled, and

with a nominal roll clearance ofsubstantially zero, CARL D QUARFORTH, Primary whereby the average rolled pellet exhibits a dlameter 10 to Width ratio of at least 1.7. REUBEN EPSTEIN: Examine"- 

1. THE METHOD OF TRANSFORMING SPHEROIDAL, ATOMIZED PELLETS OF MAGNESIUM-BASE ALLOY INTO SHEET FORM WHICH COMPRISES: PASSING THE SPHEROIDAL, ATOMIZED PELLETS BETWEEN THE ROLLS OF A MILL WHILE THE SPHEROIDAL PELLETS AND THE ROLLS ARE AT TEMPERATURES BELOW THAT AT WHICH THE PELLETS SUBSTANTIALLY STICK TOGETHER ON BEING ROLLED, AND WITH A NOMAINAL ROLL CLEARANCE OF SUBSTANTIALLY ZERO, THEREBY TO FLATTEN THE SPHEROIDAL PELLETS; PREHEATING THE SO-FLATTENED PELLETS TO A TEMPERATURE IN THE RANGE OF ABOUT 350 TO 550*C.; FEEDING THE PREHEATED, FLATTENED PELLETS BETWEEN THE HEATED ROLLS OF A MILL, THE ROLLS HAVING SUBSTANTIALLY ZERO CLEARANCE, AND THE ROLLS BEING AT A TEMPERATURE IN THE RANGE OF ABOUT 320 TO 400*C., THEREBY TO FORM SHEET MATERIAL OF MAGNESIUM-BASE ALLOY. 